Apparatus for polishing thin, flat semi-conductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is "planarized" or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. Polishing heads of the type described above used in the CMP process are shown in U.S. Pat. No. 4,141,180 to Gill, Jr., et al.; U.S. Pat. No. 5,205,082 to Shendon et al; and, U.S. Pat. No. 5,643,061 to Jackson, et al. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad, as shown in the Shendon et al patent.
Referring now to FIG. 1C, wherein an improved CMP head, sometimes referred to as a Titan.RTM. head which differs from conventional CMP heads in two major respects is shown. First, the Titan.RTM. head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16 are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
In the improved CMP head 20 shown in FIG. 1C, large variations in the removal rate, or polishing rate, across the whole wafer area are frequently observed. A thickness variation across the wafer is therefore produced as a mean cause for wafer non-uniformity. The improved CMP head design, even though utilizing a pneumatic system to force a wafer surface onto a polishing pad, the pneumatic system cannot selectively apply different pressure at different locations on the surface of the wafer. For instance, as shown in FIG. 1D, a profilometer data obtained on an 8-inch wafer is shown. The thickness difference between the highest point on the wafer and the lowest point on the wafer is almost 2,000 .ANG. yielding a standard deviation of 472 .ANG. or 6.26%. The curve shown in FIG. 1D is plotted with the removal rates in the vertical axis and the distance from the center of the wafer in the horizontal axis. It is seen that the removal rates at the edges of the wafer are substantially higher than the removal rate at or near the center of the wafer. The thickness uniformity on the resulting wafer after the CMP process is therefore very poor.
The polishing pad 12 is a consumable item used in a semiconductor wafer fabrication process. For instance, under normal wafer fab conditions, the polishing pad must be replaced after a usage of between 12 and 18 hours. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard, incompressible and thus stiffer pads are generally used to achieve planarity. Softer pads are frequently used to achieve improved uniformity and smooth surfaces. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.
A problem frequently encountered in using polishing pads in a CMP process for oxide planarization is the rapid deterioration in polishing rates of the oxide with successive wafers. The cause for the deterioration has been shown to be due to an effect known as "pad glazing" wherein the surface of the polishing pads become smooth such that the pads can no longer hold slurry in-between the fibers. This has been found to be a physical phenomenon on the surface, and is not caused by any chemical reactions between the pad and the slurry.
To remedy the pad glazing effect, numerous techniques of pad conditioning or scrubbing have been proposed to regenerate and restore the pad surface and thereby, restoring the polishing rates of the pad. The pad conditioning techniques include the use of silicon carbide particles, diamond emery paper, blade or knife for scrapping the polishing pad surface. The goal of the conditioning process is to remove polishing debris from the pad surface, reopen the pores, and thus forms micro scratches in the surface of the pad for improved life time of the pad surface. The pad conditioning process can be carried out either during a polishing process, i.e., known as concurrent conditioning, or after a polishing process.
While the pad conditioning process improves pad consistency and its lifetime, conventional apparatus of a conditioning disc is frequently not effective in conditioning a pad surface. For instance, a conventional conditioning disc for use in pad conditioning is shown in FIGS. 2A and 2B. The conditioning disc 30 is formed by embedding or encapsulating diamond particles 32 in nickel 34 coated on the surface 36 of a rigid substrate 38. FIG. 2A is a cross-sectional view of a new conditioning disc with all the diamond particles 32, 42 embedded in nickel 34. After repeated usage as a conditioning disc, the cross-sectional view of the disc 30 is shown in FIG. 2B which shows that diamond particle 42 has been lost and the top surfaces of the remaining particles 32 are flattened. The loss of diamond particle from nickel encapsulation 34 occurs frequently when the particle is not deeply embedded in the nickel metal 34. In the fabrication of the diamond particle conditioning disc 30, a nickel encapsulation 34 is first mixed with a diamond grit which included the diamond particles 32, 42 and applied to the rigid substrate 38. The bonding of the diamond particles 32, 42 is frequently insecure and thus the particles are easily lost from the nickel coating during usage. The diamond particle 42 which is lost from the nickel encapsulation 34 may be trapped between the surfaces of the polishing pad and the wafer and causes severe scratches on the wafer. Another drawback for the diamond conditioning disc is that the pad conditioning efficiency decreases through successive usage of the disc since the top surfaces of the diamond particles are flattened after repeated usage when the diamond grit mechanically abrades the pad surface.
Another processing difficulty frequently incurred in utilizing the pad conditioning disc is that while the conditioning disc may be effective in alleviating the pad glazing problem, it may not be effective in physically removing particles from the polishing pad surface, specifically, when the particles are trapped in the surface grooves. The source of the particles may be the diamond particles that have dislodged from the conditioning pad surface, coagulated or dried-up particles from the slurry solution or any other contaminating particles that may have fallen onto the polishing pad surface. The particle contamination problem becomes more serious with the continuous usage of the polishing pad since as the pad surface is gradually warned out, the depth of the grooves in the pad surface becomes smaller and thus no longer able to hold the particles therein. When the particles are released from the grooves onto the top of the polishing pad, severe scratching or other equally harmful damages to the wafer surface can occur.
FIGS. 3A, 3B and 3C are graphs illustrating the particle contamination problem on a polishing pad which is conditioned by a conventional conditioning head. For instance, FIG. 3A illustrates that at or near a pad life of 10-12.5 hours, the particle contamination problem becomes much more serious in that the failure rate doubles and quintuples those rates obtained at below the 10 hour pad life. This is a clear indication that, after 10 hours use of the polishing pad, the grooves become substantially shallower and are no longer capable of holding the contaminating particles therein. After a pad usage of more than 12.5 hours, the failure rate in wafer lots polished exceeds 50% which is clearly unacceptable. Similar trend is also seen in FIGS. 3B and 3C which illustrate the dependency of particle counts on the pad life and the dependency of particle counts on the fabrication dates, respectively. FIG. 3B shows that after a pad life of 10 hours, there is a significant increase (at a faster rate) in the particle counts. FIG. 3C illustrates the unacceptable particle counts (larger than 85) that occurred during a 20 day period obtained on a chemical mechanical polishing apparatus.
The particle contamination problem on a polishing pad surface is therefore and serious processing problem that must be resolved in order for the chemical mechanical polishing process to be used as a reliable planarization technique. In an attempt to solve the particle problem, efforts have been made to flush a polishing pad surface with high pressure deionized water jet to remove particles entrapped in the surface grooves. Other efforts have been made to manually clean the polishing pad after shutting down the chemical mechanical polishing apparatus by brushing. Neither method produces satisfactory results in obtaining a polishing pad surface that is substantially without particles. Moreover, the method either requires the complete shut-down of the polishing apparatus and thus a decrease in the fabrication yield, or requires an interruption of the polishing process in order to flush the pad with deionized water.
It is therefore an object of the present invention to provide an apparatus that is effective in cleaning a polishing pad that does not have the drawbacks and shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for cleaning a polishing pad surface and removing substantially all the particles entrapped in the surface grooves of the pad.
It is a further object of the present invention to provide an apparatus for cleaning the surface of a polishing pad that can be used in-situ without requiring down time of the polishing apparatus.
It is still another object of the present invention to provide an apparatus for cleaning particles on the surface of a polishing pad by mounting a plurality of brush means on a pad conditioning head.
It is yet another object of the present invention to provide a method for cleaning particles from the surface of a polishing pad used in a chemical mechanical polishing apparatus that can be carried out in-situ in a wafer polishing process without incurring down time of the machine.
It is still another further object of the present invention to provide a method for removing particles from the surface grooves of a polishing pad in a chemical mechanical polishing apparatus by mounting a plurality of brush means on the surface of a pad conditioning head or on the surface of a slurry delivery arm.
It is yet another further object of the present invention to provide a chemical mechanical polishing apparatus that is equipped with a slurry delivery arm and a pad conditioning head wherein at least one of the arm and the head is mounted a plurality of brush means for cleaning particles from the surface grooves of a polishing pad.